Lithographic Tuning of Polymeric Thin Film Surfaces by Stress Relaxation

We report a facile soft lithography (SL) technique that allows fabrication of patterned polymeric surfaces with feature height varying between 0 and <i>h</i>₀, using a single stamp. The method relies on the partial relaxation of the applied stress in a viscoelastic polymer thin film imprinted under a transient external load using a flexible stamp. The applicability of the technique is demonstrated for thermosetting (rubbery) as well as thermoplastic (glassy) polymers over a wide range of lateral dimensions. The lateral dimension and the periodicity of imprinted patterns remain identical to that of the original stamp. The method has potential applications in creating templates for performing combinatorial experiments related to wetting and dewetting studies, adhesion, nanotribology, microfluidics, etc

Solvent-Vapor-Assisted Dewetting of Prepatterned Thin Polymer Films: Control of Morphology, Order, and Pattern Miniaturization

Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (<i>h</i>_f). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (<i>h</i>_rm) is lower than a limiting thickness <i>h</i>_lim ≈ 10 nm. Additionally, the width of the pattern grooves (<i>l</i>_s) must be wider than λ_s corresponding to a flat film having a thickness of <i>h</i>_rm for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (<i>h</i>_f), prepattern amplitude (<i>h</i>_st), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh–Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same <i>h</i>_f

Thermally Tailored Gradient Topography Surface on Elastomeric Thin Films

We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (<i>L</i>), resulting in a surface with a gradient topography with progressively varying feature heights (<i>h</i>_F). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting

Creation of self-organized complex meso patterns in sol-gel thin films by confined capillary dynamics

We report a simple and facile technique for creating complex mesoscale patterns with two-dimensional (2-D) orders on the surface of a sol gel derived silica thin films using a combination of a topographically patterned stamp and a substrate, each having a one-dimensional (1-D) grating structure (stripes). The self-organized pattern formation results as a consequence of capillary flow of the sol film in liquid state under a confining stamp. The key novel aspect of the work lies in using a topographically patterned substrate, dip coating on which results in a film with an undulating top surface. When a grating stamp is brought in conformal contact with such a film in a manner so that the direction of the stamp stripes are at right angles to the direction of the undulations on the film surface, a periodically ordered spatial variation in the extent of contact between the film and the stamp results. Subsequent capillary flow of the liquid sol along the stamp walls occurs only at the locations where the stamp surface and the film are in direct contact, resulting in an array of pillars with 2-D order. We further show that the morphology of the final patterns can be tailored by varying various parameters such as the initial film thickness, wettability of the stamp, and periodicity of the substrate. We further discuss the gradual oxidation of the sol film with progressive thermal annealing and the associated shrinkage of the structures in terms of pattern dimensions. Generation of meso patterns with 2-D order on an inorganic sol gel film surface using a stamp and a substrate, each having strictly 1-D features, is the key novel aspect of the work, as it is a rare example when patterns other than a mere negative replica of a stamp are created on a sol gel thin film by any soft lithography technique

Capillary Force Lithography Pattern-Directed Self-Assembly (CFL-PDSA) of Phase-Separating Polymer Blend Thin Films

We report capillary force lithography pattern-directed self-assembly (CFL-PDSA), a facile technique for patterning immiscible polymer blend films of polystyrene (PS)/poly­(methyl methacrylate) (PMMA), resulting in a highly ordered phase-separated morphology. The pattern replication is achieved by capillary force lithography (CFL), by annealing the film beyond the glass transition temperature of both the constituent polymers, while confining it between a patterned cross-linked poly­(dimethyl siloxane) (PDMS) stamp and the silicon substrate. As the pattern replication takes place because of rise of the polymer meniscus along the confining stamp walls, higher affinity of PMMA toward the oxide-coated silicon substrate and of PS toward cross-linked PDMS leads to well-controlled vertically patterned phase separation of the two constituent polymers during thermal annealing. Although a perfect negative replica of the stamp pattern is obtained in all cases, the phase-separated morphology of the films under pattern confinement is strongly influenced by the blend composition and annealing time. The phase-separated domains coarsen with time because of migration of the two components into specific areas, PS into an elevated mesa region and PMMA toward the substrate, because of preferential wetting. We show that a well-controlled, phase-separated morphology is achieved when the blend ratio matches the volume ratio of the elevated region to the base region in the patterned films. The proposed top-down imprint patterning of blends can be easily made roll-to-roll-compatible for industrial adoption

Differential Behavior of Normal and Fibrotic Fibroblasts under the Synergistic Influence of Micropillar Topography and the Rigidity of Honey/Silk-Fibroin Substrates

We report differential proliferation behavior of normal and fibrosis associated human oral fibroblasts on micropillar honey embedded silk fibroin substrates (HSF). Oral fibroblasts of different origins manifest differences in proliferation rate, morphology, and the cytoskeletal arrangement on HSF substrates with distinct topography (H, D, and S), stiffness, and honey concentration. It is observed that the proliferation rate is maximized for normal and inhibited for fibrosis associated fibroblasts on a HSF substrate surface with moderate height of ∼8.5 μm and 2% honey concentration. Molecular expression analysis reveals decrease in c-myc and p53 expression in later cells validating the inhibition of their proliferation rate, which is further correlated with the decreased Col I and Col III expression on this substrate. A substrate with enhanced interspacing and intermediate mechanical stiffness (0.57 ± 0.32 μN/nm) favors strong adhesion and stable cell–matrix interaction for normal cells, while exhibiting negative influence on fibrotic fibroblasts with poor adhesion and spreading capability. Decrease in vimentin, fibronectin expression, and cytoskeleton reorganization justify the poor stability of later cells on the optimized substrate, thereby allowing selective modulation of normal and fibrosis associated fibroblasts under the synergistic influence of honey concentration, topography, and rigidity of HSF substrates. The work highlights the possible therapeutic efficacy of honey based micropatterned substrates as smart patches for fast wound healing and in minimizing the chances of recurrence of precancer post oral tumor resection surgeries